Wasting diseases may be categorised into generalised and localised wasting diseases. To deal first with generalised wasting, many disease processes can lead to aggressive generalised weight loss through either the inability to consume sufficient nutrients and energy sources, through their loss from the body (either enterally or in the form of cellular matter), or through an inability to absorb them. Other diseases are associated with marked weight loss quite out of proportion to any reduction in nutrient absorption or increase in nutrient loss. Such weight loss may have a metabolic origin. Severe cardiac failure as well as renal, hepatic and malignant disease processes are all associated with such inappropriate weight loss. Some neurological diseases, such as Parkinson's disease and syndrome are similarly related, as are conditions associated with inflammatory processes, such as severe sepsis or septic shock and autoimmune and connective tissue disorders. This weight loss may at best be disabling, and at worst associated with an increased mortality. Current treatment and preventative strategies largely focus on nutritional support.
In localised wasting, disuse of any given muscle group (for instance due to musculoskeletal or neurological injury) may lead to wasting in the affected territory. There are currently no available treatments which are routinely used to slow or limit such wasting, nor which have been shown to accelerate the reversal of such wasting with appropriate exercise or after the cessation of the initiating disease state.
Current strategies for the promotion of trainability and fitness have largely focused on alterations in training pattern. More recently, nutritional supplementation has been suggested using the manipulation of scale and nature of intake of carbohydrates, fats, vitamins and amino acids. The addition of other substrates, such as creatine derivatives, have also been used. Most such interventions are either currently unproven, or have been shown to have no or only modest influence. Endocrinological interventions have been attempted, including the use of androgens and other steroid hormones. The use of insulin or of growth hormone may also have a role. However, these treatments may be associated with an unacceptable side-effect profile and also suffer from the disadvantage that they have to be parenterally administered (usually by intramuscular injection). Pharmacological manipulations are not currently available.
The possibility of improving cardiovascular, and other organ, function is known in connection with the phenomenon of “preconditioning”. The exposure of an organ—most notably the heart—to a brief period of reduced blood flow or oxygen supply has been shown to provide protection against a second more severe similar event which might otherwise prove lethal to cells or the organ itself. Much research is currently being undertaken in an effort to identify pharmacological agents which might mimic this process. None is available for routine clinical practice.
The renin-angiotensin system (RAS) and its components may be described as follows. Briefly, cells of the renal juxta-glomerular apparatus produce the aspartyl protease renin which acts on the alpha-2 globulin angiotensinogen (synthesised in the liver) to generate angiotensin I (AI). This non-pressor decapeptide is converted to angiotensin II (ATII) by contact with the peptidyldipeptidase angiotensin-converting enzyme (ACE) (reviewed in (1)). ATII stimulates the release of aldosterone, and is also a potent vasoconstrictor. The renin-angiotensin system is therefore important in the maintenance and control of blood pressure as well as the regulation of salt and water metabolism. Renin, angiotensinogen and ACE have also been identified in cardiovascular tissues including the heart (2) and blood vessels, as has mRNA for components of this system such as angiotensinogen (3-5). Receptors for angiotensin II have been found on vascular smooth muscle cells (6). Within tissues, the RAS may therefore have a local paracrine function (reviewed in (7, 8)), and the expression of the different components can be altered by pathophysiological stimuli such as sodium restriction (5). Kinetic studies suggest that much of the circulating angiotensin I and II is derived from the both renal and non-renal tissues (9-11).
ACE is a zinc metallo-protease which catalyses conversion of the inactive decapeptide ATI to the active octapeptide ATII thorough the hydrolytic cleavage of dipeptides from the carboxyl terminus His-Leu dipeptide. It also catalyses inactivation of bradykinin (a patent vasodilator) by two sequential dipeptide hydrolytic steps; in this context, ACE is also known as kininase II.
The presence of renin-angiotensin system (RAS) components in many animal species (such as locusts and elasmobranchs) suggests that they must have some other role than that of a conventional circulating RAS. This function must be fundamental and important in order to have been phylogenetically conserved over many millions of years. In fact, complete renin-angiotensin systems are now thought to exist within many human (and animal) tissues: physiologically-responsive gene expression of RAS components within these tissues, local generation of ATII, the presence of ATII receptors and the demonstration that these receptors are physiologically active have all been shown. Thus, angiotensinogen messenger RNA (mRNA) is identified in renal neural and vascular tissues, and local synthesis may strongly influence its concentration in interstitial fluid (10). Renin mRNA (12) and product (13) is found in cultured mammalian vascular smooth muscle cells and throughout the vessel wall (13), and in rat ileum, brain, adrenal, spleen, lung, thymus and ovaries. Liver renin gene expression is physiologically responsive, being increased 3-fold by sodium deprivation or captopril administration (14).
Non-renin angiotensinogenases may also exist in tissues. A neutral aspartyl protease with renin-like activity has been demonstrated in canine brain (15, 16). Some (e.g. tonin, elastase, cathepsin G and tissue plasminogen activator) can cleave ATII directly from angiotensinogen (16).
ACE expression occurs at high level in vascular endothelium, but also in the small intestinal epithelium, the epididymis (17) and brain (15). Tissue-specific/age-related ACE gene transcription occurs in renal tissue (where there is very high proximal tubular epithelial expression), and in cardiovascular, hepatic and pulmonary tissues (18).
Such local systems may be paracrine in nature: receptors for ATII are classically described as existing on cell surfaces, allowing transduction of the effects of endocrine and paracrine ATII. However, true autocrine systems (intracellular production and actions) may also exist. ATII receptors may also exist on the cell nuclei. Specific binding sites for ATII exist on cellular chromatin which may regulate gene transcription (19, 20).
There are many marketed or investigation-stage agents which inhibit RAS activity, and many of them fall into two broad classes: the inhibitors of angiotensin-converting enzyme, whose approved names generally end in “-pril” or in the case of active metabolites “-prilat”, and antagonists at angiotensin receptors (more specifically, currently, the AT1 receptor), whose approved names generally end in “tartan”. Also potentially of increasing importance may be a class of drugs known as neutral endopeptidase inhibitors, some of which will also have an ACE-inhibitory effect or the potential to reduce RAS activity.
Brink et al. (21) suggested that angiotensin II may have a metabolic effect in rats (in vivo experimental work) which is independent of its effects on blood pressure.
There is evidence that angiotensinogen gene expression is differentially modulated in fat tissue in obese rats when compared to their equivalent lean strain (22).
ACE inhibition increases rabbit hind leg oxygen consumption at high work loads, but not at lower workloads (23).
ACE inhibitor (ACEI) increases insulin-dependent glucose uptake into the skeletal muscle of an obese rat strain which exhibits relative insulin-resistance (24), and this may be kinin-dependent (25). Glucose transporter levels were elevated in this study, as they were sustained by AT1 receptor antagonism in the diabetic rat heart (26).
ATII increases rat hind limb O2 usage and twitch tension (27). This paper concludes that the effects might have been due to effects on blood flow or neurotransmission and not to a direct metabolic effect.
In heart failure in dogs, fatigue-resistant fibres are conserved by ACE inhibitor therapy (28). In rats, capillary density is maintained, and collagen volume reduced (29, 30).
Kininases (such as ACE) have been shown to exist in the cell membranes of human skeletal muscle (31). Thus, skeletal muscle RAS may exist (32).
In vitro, ACE inhibitors cause an increase in myocardial oxygen utilisation. Whether this was due to increased or reduced efficiency was unclear (33). This work related to myocardial muscle extracts. This effect may be due to reduced knin breakdown, and thus increased kinin levels, despite the fact that angiotensin II may modulate (and increase) kinin release (34).
Other publications suggest an effect of ACE inhibitors or of angiotensin II on muscle performance or metabolism, but all of these have concluded that the effects are mediated by alterations in nutritive blood flow (35, 36).
In human forearm, kinins increase blood flow and glucose uptake, although again a direct effect of RAS, or an effect on performance, was not detailed (37).
Losartan (an AT1 antagonist) improves insulin sensitivity in human skeletal muscle (38).
Other publications suggest no beneficial effect of ACE inhibition, amongst those with heart failure in muscle energy balance (39). ACE inhibition did not alter perceived work or maximal work capacity of 20 students on a bicycle ergometer (40).